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Date: Mon, 10 Sep 2012 22:05:13 +0200
From: wolf_dl4yhf <dl4yhf@freenet.de>
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To: RSGB LF Group at blacksheep <rsgb_lf_group@blacksheep.org>
CC: RSGB LF Group at Yahoo <rsgb_lf_group@yahoogroups.co.uk>
Subject: LF: Small "linear" medium wave PA
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Hello Uwe,

I have completed the 2nd part of the circuit diagram of my small 
"linear" PA :

http://www.qsl.net/dl4yhf/mf_reception/MF_Reception.html#mf_linear_pa

You may have to reload the page (press F5) since it has just been uploaded.

IMO, the efficiency for this power range (50 ... 60 watts in my case) is 
adequate. It's simple, and even when "slightly overdriven" it retains 
the amplitude ramp of the keying signal from the driving transmitter (IC 
706 with 2 watts output on MF). When driven to the clipping point, which 
is OK for CW, the amplifier drew just above 3 amps DC at 13.8 Volts to 
produce 30 watts of RF. At that power level, a small heatsink ("CPU 
cooler") is sufficient.

With more turns on the drain output transformer, and a larger heatsink, 
even more power would be possible, possibly up to 100 watts (which I 
don't need at DF0WD).

The DC overcurrent protection is simple but effective: With the two 
transistors on the 2nd part of the circuit, the gate bias voltage is 
simply "pulled down" so the drain current and power dissipation stays in 
non-lethal limits (for the MOSFETs). The 30 milliohms resistor is just a 
short piece of 'Constantan' wire. It is barely visible on the photo of 
the board, near the "DC current sense / limiter".

All the best,
   Wolf .